Abstract: Rechargeable molten-salt batteries represent a promising technology for large-scale energy storage, offering intrinsic safety, high power density, and low cost, with their key advantage lying in molten-salt electrolytes that are nonflammable and highly conductive; unlike flammable organic liquid electrolytes with limited thermal stability and ionic conductivity, molten salts avoid volatile solvents, reducing fire and explosion risks while enabling fast charge and discharge, and many can be made from abundant, inexpensive materials, making them attractive for grid-scale storage where safety, cost, and longevity matter. However, challenges remain, including limited energy density, capacity degradation, and parasitic side reactions such as corrosion, and corrosion of current collectors, seals, and housings, along with electrode dissolution and unwanted interfacial reactions, shortens cycle life and lowers efficiency, while high operating temperatures can accelerate degradation and require robust thermal management, insulation, and hermetic sealing, and freeze-thaw cycling, material compatibility, and durable components further complicate deployment. This review provides a comprehensive overview of the historical development of molten-salt batteries, emphasizing representative chemistries including thermal primary cells, Na-S, Na-NiCl₂, liquid metal, metal-air, low-temperature Al-based, and dual-intercalation systems; early thermal primary cells, such as Li-FeS₂ reserve batteries, proved molten-salt electrolytes could work in harsh conditions, though they were not rechargeable, while Na-S batteries offered high energy density and low-cost materials but required high temperatures and faced safety and sealing issues, Na-NiCl₂ or ZEBRA batteries improved safety and enabled moderate-temperature operation, liquid metal batteries introduced all-liquid electrodes and electrolytes, supporting high current density and long life for stationary storage, metal-air systems, especially Fe-air, promise ultra-low cost but suffer from low round-trip efficiency and air-electrode challenges, and low-temperature Al-based and dual-intercalation systems expand the design space by reducing operating temperatures and exploring multivalent or dual-ion chemistries. We highlight the evolution in four distinct configurations, including high-temperature cells with solid ion conductors (i.e., ceramic), liquid metal batteries, solid-electrode systems with low-melting-point electrolytes, and metal-air designs. These configurations offer diverse performance trade-offs, from high-energy-density Li-metal cells (~400 Wh kg^-1) to ultra-low-cost Fe-air systems (<100 $ kWh^-1), and each balances energy density, power density, cycle life, safety, and cost differently, making molten-salt batteries suitable for differentiated niches rather than one universal application. Despite persistent challenges such as high viscosity, poor wettability, and the need for scalable thermal management and durable components, low-melting-point molten salts provide safer alternatives to flammable organics. Liquid metal batteries show particular promise for grid-scale storage, while molten-salt metal-CO₂ cells uniquely couple energy storage with carbon utilization, and overcoming these interdisciplinary hurdles, spanning materials science, electrochemistry, thermal engineering, and system integration, positions molten-salt batteries to become suitable technologies for resilient, renewable-powered energy grids, so that by addressing these challenges they are well positioned to transition from compelling laboratory prototypes to cornerstone technologies, occupying differentiated niches that range from bulk, low-cost storage to high-performance, high-value applications.

Key words: Molten salt battery, Thermal cell, Liquid metal cell, Molten salt electrolyte, Energy storage